1. Field of the Invention
The present invention deals with a process for the preparation of uretdione group containing polyisocyanates comprising reacting monomeric aromatic diisocyanates in the presence of a catalyst.
2. Description of the Related Art
It is basically known to prepare uretdiones by reacting isocyanates in the presence of certain catalysts.
DE-A-3 739 549 discloses a process for the preparation of cycloaliphatic uretdiones in which cycloaliphatic diisocyanates are used as a starting material and pyridine is used as a catalyst.
Richter and Ulrich in Synthesis, 1975, pg. 463 describe a process in which benzyl isocyanate is reacted in the presence of 1,2-dimethylimidazole used as a catalyst. However, in this process, a large amount, about 20 percent, of isocyanurate byproducts are obtained. The presence of isocyanurates is undesirable for a number of applications since these are trifunctional and have a tendency to crosslink. Noack and Schwetlick in Z. Chem., 26 (1), pg. 117, 1986 also report this disadvantage of the formation of isocyanurates. When reacting a mixture of 80 weight percent 2,4-toluene diisocyanate and 20 weight percent of 2,6-toluene diisocyanate with 1-methylimidazole, 1,2 -dimethylimidazole or 1-butyl imidazole as catalysts in a mixture of methylethyl ketone and cyclohexane only a maximum of 51% dimer yield is obtained.
Polish patent 100 148 describes reacting aromatic diisocyanates with 1-allyl-2-methylimidazole as a catalyst. However, here the dimer yield is only 50%.
The object of the present invention was to find a process for the preparation of uretdione group containing polyisocyanates which reduces the formation of isocyanurates as far as possible.
The object was met by reacting monomeric aromatic diisocyanates in the presence of a catalyst whereby imidazoles or benzimidazoles having general structural formulas I or II are used as said catalyst: ##STR2## or mixtures thereof are used as said catalysts, whereby R.sup.1 and R.sup.5 independently are hydrogen atoms; C.sub.1 - to C16- alkyl groups; alkenyl or alkynyl groups having 2 to 16 carbon atoms; aryl groups; alkylaryl or aralkyl groups, whereby the alkyl groups can have from 1 to 10 carbon atoms; amino groups; hydroxy groups or mercapto groups, aminoalkyl groups; alkylamino groups; alkoxy groups; oxyalkyl, alkylthio groups or thioalkyl groups having C.sub.1 - to C16-alkyl groups; or halogen atoms, and R.sup.2, R.sup.3, R.sup.4, R.sup.6, R.sup.7 have the same meaning as R.sup.1 and R.sup.5 or are nitro groups; with the proviso that the following compounds are excluded:
1-allyl-2-methylimidazole, PA0 1,2-dimethylimidazole, PA0 1-methylimidazole, and PA0 1-butylimidazole.
The preferred embodiments of the present invention are found in the dependent claims.
It has also been shown that the content of isocyanurates lies below about 2% both when using a water containing solvent as well as an anhydrous solvent.
Typical monomeric aromatic diisocyanates are, for example, 1,5-naphthalene diisocyanate, 4,4'-diphenyl diisocyanate, 1,4-phenylene diisocyanate, as well as more preferably 2,6-toluene diisocyanate, polymethylenepolyphenylenepolyisocyanate, and most preferably 2,4-, and 2,6-toluene diisocyanate and 2,2'- 2,4'- and 4,4'-diphenylmethane diisocyanate.
The reaction can be done neat or in the presence of an inert organic solvent whereby the latter embodiment is preferred. Most preferred is carrying out the reaction in an inert organic solvent with a water content of from 50 to 500 ppm. Toluene, methylethylketone, and n-hexane are examples of suitable solvents. The degree of dryness of the solvent is of great importance. When using commercially available toluene having a water content of about 300 to 500 ppm or commercially available n-hexane having a water content of about 200 to 300 ppm, dimer yields of over 90% are obtained and the product is slightly contaminated by urea. If on the other hand an anhydrous solvent is used having a water content less than 100 ppm the yields of uretdiones are indeed somewhat smaller but the product is almost completely free of ureas. When using both a water-containing as well as anhydrous solvent one obtains a content of isocyanurates less than 2 percent.
The weight ratio of solvent to diisocyanate ranges from 0.1:1 to 10:1. Preferred is using a ratio of from 0.3:1 to 3:1.
The catalyst, likewise in solution, is added to the dissolved diisocyanate at temperatures of from -10.degree. C. to 80.degree. C. more preferably 20.degree. C. to 50.degree. C. while stirring.
Imidazoles I or benzimidazoles II used as catalysts according to the present invention can be used individually or in mixtures. Mixtures of I and II are also possible. Proven most suitable are 1,2,4,5-tetramethylimidazole,1-(n-butyl)-2,4,5-trimethylimidazole and 1-benzyl-2-methylimidazole. The catalysts are used in quantities of from 0.01 to 50 weight percent, more preferably 0.05 to 1 weight percent based on the diisocyanate. The weight ratio of solvent to imidazole and/or benzimidazole ranges from 1:1 to 100:1, more preferably 10:1 to 60:1.
The resulting reaction product is washed and dried.
The advantages achieved with the instant invention are that the content of undesirable isocyanurate lies below 2 percent and that the purity and yield of the uretdiones can be influenced with the reaction in solution via the degree of dryness of the solvent.
The uretdione group containing polyisocyanates prepared in this fashion can be used to prepare polyurethanes.